This invention relates to methods and apparatus for sensing the level of a fluid, either solid particulate or liquid, in an enclosed container or tank. More particularly, the invention relates to such a tank level indicator system employing the ultrasonic transmission of acoustic of vibratory energy for indicating fluid level.
In oil field fracturing and cementing operations large tanks or containers of fluids are connected to high pressure mixing and pumping systems. Such pumping systems are used to inject the fluids, or mixtures thereof, under pressures ranging up to 25,000 psi into a wellbore or earth formations surrounding a well borehole. Fluids from a battery of multiple tanks may be pumped in parallel to mixing apparatus or to high pressure staging pumps for injection into the subsurface environment. The fluids in different ones of the multiple tanks may be utilized at different rates in the mixing or pumping apparatus. The tanks will not empty at the same rate during such an operation. It is extremely important in such operations to know the level of fluid in a given tank at any instant of time. Catastrophic failures of high pressure pumping apparatus can occur if a tank is completely emtpied unknowingly, and the high pressure pump allowed to suck air into the system.
It will be appreciated by those skilled in the art, that various fluids, both solid and liquid, and some of which may be of a non-Newtonian nature can be utilized in oil well fracturing and cementing operations. Froths or foams may accumulate on the surface of such liquid materials in tanks awaiting usage in such operations. Similarly, bubbles of air or gas may be present in the fluid. It will also be appreciated by those skilled in the art, that extreme vibratory or acoustic noise exists in such environment due to the energy dissipated by pump motors and heavy equipment in the vicinity. All of these factors combine to provide a very hostile environment in which measurements of the fluid level in a tank must be satisfactorily made.
Attempts to measure the fluid level in such tanks by the use of sight glasses or floating mechanical indicators have proven to be unreliable. The present invention, by use of ultrasonic transmitting and receiving apparatus which may be applied to the exterior surface of a tank, provides a system which can accurately monitor the level of fluid inside the tank under the hostile environmental conditions present in oil field operations. The present invention provides a unique and accurate approach to the measurement of fluid levels in a tank by use of the propagation velocity of acoustic or ultrasonic energy in the walls of such a tank.